The present invention relates to semiconductor device fabrication, and more specifically, to the fabrication of high aspect ratio structures.
Micro-electro-mechanical system (MEMS) manufacturing often requires the fabrication of high aspect ratio structures. One of the primary challenges in MEMS manufacturing is the ability to etch uniform high aspect ratio structures across a substrate surface. For example, profile asymmetries as low as 10 nm have been observed to produce unacceptable device performance in a MEMS gyroscope element. Etch non-uniformities may exhibit an aspect ratio dependence, a micro-loading or pattern density dependence, an across-substrate dependence, or an etch chamber dependence.
Critical dimension features near the bottom portion of the high aspect ratio structures are difficult to measure with an in-line critical dimension (CD) scanning electron microscope (SEM) because shadowing of the primary electron beam occurs beyond a certain depth at a given feature aspect ratio. While dimensions of the top portion of the high aspect ratio structures can be readily measured using an SEM, the dimensions of the bottom portion of the high aspect ratio structures is arguably more important. Specifically, the bottom portion of the high aspect ratio structures determines the feature size uniformity of subsequently etched features if the high aspect ratio structures are used as a hardmask, or the device performance depends on the original high aspect ratio structures. An inaccurately measured critical dimension influenced by the depth of the high aspect ratio structures may produce errors if used to adjust either the photolithography process or a subsequent etching process.
Therefore, there is a need for processing methods that improve the dimensional uniformity of high aspect ratio structures.